Technical Bulletin: Ground and Floor Surfaces

Measuring slip resistance involves the minimum tangential force necessary to initiate sliding of a body over the surface and the body gravity force. The coefficient of friction between the two surfaces is the ratio of the horizontal and vertical forces required to move one surface over another to the total force pressing the two surfaces together.

There are three critical stages in an individual's gait: 1) touchdown, 2)​ full load, and 3) push-off. In order to avoid slippage while walking, the horizontal and vertical forces applied by the individual must be resisted by forces acting against the foot as it contacts the walking surface. The definitive component of this resisting force, and the variable most subject to manipulation, is the coefficient of friction of the surface material. Consider, for example, an icy surface with a negligible coefficient of friction. A runner whose forward motion applies a substantial horizontal force will slip-and probably fall-on such a surface. A more careful pedestrian may be able to limit his horizontal force contribution so that it balances the available frictional resistance of the ice and thus cross it safely. Adding sand to the icy surface will increase its coefficient of friction and allow for a more standard gait. Once the ice has melted, the higher coefficient of friction of the newly-exposed surface will offer sufficient resisting force to permit the runner to speed across it without incident.

The dynamic coefficient of friction varies in a complex and non-uniform way. Although R can be calculated and modeled in the laboratory using sophisticated computer programs, the more straightforward measurement of the static coefficient of friction provides a reasonable approximation of the slip resistance of most surfaces and is the method most appropriate for evaluating surface materials and finishes.

A variety of devices are available for such measurements. The most common device, the James machine, was developed in the early 1940s and was the testing device specified by the Underwriters Laboratory (UL) shortly thereafter when it established--from laboratory test data corroborated by field experience--a minimum value of 0.5 for the static coefficient of friction for floor polish bearing the UL seal. Since then, 0.5 has become the commonly-accepted threshold for classifying slip resistance in products. Furthermore, the James machine is the  recognized test method and the 0.5 value (when measured by this  tester) is the recognized minimum criterion for slip-resistant walking surfaces in courts of law in the United States.

Measurement by the James machine, utilizing a leather sensor, is the only method appropriate for assessing surfaces and products against the 0.5 UL standard for static coefficient of friction. Using a different sensor material, even If measured by the James machine, will give a different reading for the same surface material.

This is a significant point. An informal comparison of data collected under three different research protocols, involving four different friction-testers and four different shoe sensor materials, all applied to the same 8-inch  by 8-inch ceramic tile surface, resulted in thirty readings ranging from a low of .29 to a high of .99-for its static coefficient of friction. Even  limiting values to those measured by the James machine but using both leather and Neolite sensor material resulted in a range of 0.57 (leather) to 0.79 (Neolite) for the same surface being tested.

It is impossible to correctly specify a slip-resistance rating without identifying the testing method, tester, and sensor material to be used in evaluating the specified product and equally invalid to compare values obtained through one methodology to those resulting from different testing protocols. Because a consensus test protocol has not yet been identified, the Access Board did not specify a value or testing method for determining the coefficient of friction along an accessible route.

The James machine continues to be a laboratory mainstay, but is not portable and thus cannot be used in field testing. In order to measure the slip-resistance of surfaces already in place, researchers at The Pennsylvania State University evaluated three portable testers: the NBS-Brungraber Tester (also known as the Mark I Slip Tester), the PTI (Pennsylvania Transportation Institute) Drag Sled Tester, and the Horizontal Pull Slipmeter.